Mobile radio modes known at present to the person skilled in the art include the second generation Global System for Mobile communication (GSM) mode, the third generation Universal Mobile Telecommunication System (UMTS) mode, and the Personal Communication Service (PCS) mode in use in America, for example.
Conventionally, a mobile radio terminal sets up calls via base stations using a radio access technology (RAT). Different radio access technologies can coexist, with base stations dedicated to a first mobile radio mode, for example GSM base transceiver stations (BTS), and stations dedicated to a second mobile radio mode, for example UMTS Terrestrial Radio Access Network (UTRAN) antennas. Other radio access technologies may be introduced in the future, with other mobile radio modes, and become part of the problem addressed by the invention.
Given this background, it would appear to be crucial that a mobile radio terminal be able to operate in accordance with any standard associated with any current mobile radio mode, i.e. with any of the radio access technologies associated with said modes. This interworking is essential for three main reasons: to maintain continuity of service during a call, to maintain continuity of service between calls, when the terminal is in standby mode, and to support measurements.
To set up a call, a mobile terminal must be associated with a base station of the geographical area in which it is located, which may be dedicated to any of the radio access technologies currently in use in the territory concerned. The base station passes the call to a controller which passes it to a switching center which directs the call to the base station associated with the destination terminal or to another network associated with the destination terminal (Internet, public switched telecommunication/telephone network (PSTN), etc.). If the mobile terminal moves around while a call is in progress, handover is required to maintain continuity of service. The handover can be between base stations using the same or different radio access technologies.
Between calls, a mobile terminal must remain in a standby state, in which it is also associated with a base station. If it is moving around, the mobile terminal can perform a reselection operation to connect to a base station in a new geographical area. Reselection is also required if the mobile terminal is communicating in a data packet mode, for example if it is using the General Packet Radio Service (GPRS).
Finally, a multimode mobile radio terminal must be able to support regular measurement of signals received from a base station using a first mode (for example the GSM mode) even though it is connected by a radio link to a base station using a different mode (for example the UMTS mode). Such measurements are often imposed by the standardization bodies that regulate existing and projected mobile radio services.
Interworking of a plurality of radio access technologies (for handover, reselection, measurements) is not necessarily guaranteed, in particular because each mobile radio mode has its own clock. Thus in the case of the GSM and UMTS modes, the periods of the radio frames for calls are not the same, being 4.615 ms for the GSM and 10 ms for the UMTS. This is not merely a question of lack of synchronization between base stations, but a real difference in the time frame of reference between the two standards.
A multimode mobile terminal connected by a radio link to a base station using a first mode must therefore be in a position to know the exact time for at least one other mode. It is therefore important for a multimode mobile terminal to be able to determine the time shift between two mobile radio modes in order to be able to switch from one to the other during a call, when in standby mode, or to carry out measurements.
One solution is to continue to listen regularly to one mode while using another mode (for example, listening to GSM stations when communicating with UMTS stations). This kind of solution is not the optimum, however, because it is costly in terms of energy consumption and therefore in mobile terminal battery life.